1. Field of the Invention
The present invention relates to a demultiplexer that can divide a multiplexed signal, generated by superposing a plurality of pulsed signals with carriers having mutually different frequencies, according to the respective carrier frequencies. The present invention also relates to a multiplexer that can superpose those pulsed signals one upon the other.
2. Description of the Related Art
Recently, the technique of transmitting a multiplexed signal generated by superposing a plurality of pulsed signals with carriers having mutually different frequencies has attracted much attention in the art. To transmit such a multiplexed signal, a demultiplexer and a multiplexer are needed.
FIG. 22 shows a conventional demultiplexer including circulators and bandpass filters. A demultiplexer of this type is disclosed by Yoshihiro Konishi in “Fundamentals and Applications of Microwave Circuits—from Basics to New Developments”, 3rd edition, published Jun. 10, 1995, p. 306, FIG. 8.16.
In the demultiplexer shown in FIG. 22, circulators 192 are connected on multiple stages along a single line. A bandpass filter 193 branches from the circulator 192 on its associated stage. In FIG. 22, a number n of bandpass filters 193 are used.
An RF signal, incoming through an input port 191, is guided first by the circulator 192 on the initial stage to the bandpass filter 193 on the initial stage. The bandpass filter 193 on the initial stage has a passband F1 and outputs only selected frequency components of the RF signal received, falling within the passband F1, to an output port 194 on the initial stage. Meanwhile, the rest of the RF signal, i.e., the remaining frequency components thereof falling out of the passband F1, is guided by the circulator 192 on the initial stage to the circulator 192 on the next stage. This RF signal is output by the bandpass filter having a passband F2 to the output port.
Similar wavelength division is carried out on the third stage and so on. In this manner, the RF signal, in which a number of frequency components have been superposed one upon the other, can be divided according to desired passbands Fi (where i=1, 2, . . . , and n).
Depending on the specifications of a given communication system, the demultiplexer may need to exhibit low loss and steep filtering characteristic. When the conventional demultiplexer is used in a milliwave communication system, a waveguide filter is often used as the bandpass filter 193.
A waveguide filter exhibits low loss in its passband and excellent attenuation ability in its stopband. However, it is hard to reduce the size and weight of a waveguide filter. Accordingly, if such a filter is used in a demultiplexer that needs a plurality of filters as shown in FIG. 22, then the demultiplexer will have an increased size and a higher price.
In order to overcome the problems described above, a primary object of the present invention is to provide a less expensive demultiplexer and multiplexer, which can effectively contribute to size and weight reduction.